This invention relates in general to a process for purifying an aldehyde and, more particularly, to a method of purifying an alkyl glyoxylate by decomposing its hemiacetals and hydrates and removing the liberated water and/or alcohol.
High purity alkyl glyoxylates are important in the synthesis of pharmaceuticals and various polymers. In order to manufacture long-chained polyacetals, i.e., polyacetals with molecular weights of at least 8800, the water and alcohol content (expressed as monomeric species) of the aldehyde must be low, i.e., usually less than 1000 ppm (w/w). The maximum chain length of the polyacetal is approximately equal to the molar ratio of the aldehyde to water plus alcohol, all expressed as monomeric species.
Alkyl glyoxylates can be produced by the oxidative dehydrogenation of alkyl glycolates in the presence of a silver catalyst. For each mole of glyoxylate produced, about one mole of water is co-produced, along with lesser amounts of the alcohol corresponding to the alkyl glyoxylate and various other minor by-products, i.e., glyoxylic acid. Product isolation to the above purity levels by separation processes such as distillation is complicated by reactions between the alkyl glyoxylate and hydroxylic species such as water and the alcohol corresponding to the alkyl glycolate.
Kinetic studies show that the equilibrium constants for the hydrate and hemiacetal formation are large. Thus, alkyl glyoxylate containing small amounts of water and alcohol is difficult to purify, since the water is tied up as the hydrate, and the alcohol is tied up as the hemiacetal. Therefore, separation of any hydroxyl group from the alkyl glyoxylate cannot be achieved by simple distillation and/or inert gas stripping.
U.S. Pat. No. 4,502,923 discloses a method of isolating alkyl glyoxylate from its impurities in a series of distillations. First, the water and alcohol content is lowered by vacuum distillation in a first column. The residue from the first column is distilled in a second vacuum column to decrease the ratio of alkyl glycolate to alkyl glyoxylate. The residue from the second column is then distilled in a third column at a higher pressure to recover the high purity alkyl glyoxylate as an intermediate distillate. A low boiling azeotroping agent, such as methylene chloride, is introduced to cause water and alcohol to distill overhead. In order to produce high purity alkyl glyoxylate, it is necessary to have a large recycle stream on the order of 11 lbs. of recycle alkyl glycolate throughout the three distillation columns per pound of product alkyl glyoxylate takeoff. In this scheme, the capital investment in plant equipment is high, energy consumption is high, and the production of product is relatively low.
Australian Patent No. 30007-84 discloses a method for isolation of the glyoxylic ester from materials coformed during the oxidative dehydrogenation of an alkyl glycolate. Immediately after leaving the reactor, the gaseous reaction mixture is quenched with a low-boiling entrainer, such as a hydrocarbon having a lower boiling point than the glyoxylic ester. The entrainer is used to azeotropically distill water and alcohol. The reaction mixture, along with the entrainer, are passed to a rectifying column, where the water azeotrope and other low boilers are taken overhead while the glyoxylic ester is taken off as bottoms. Some of the water and alcohol in the reaction mixture forms high-boiling hemiacetals and hydrates before they can be azeotropically distilled. Consequently, additional steps are required to obtain high purity alkyl glyoxylate.
It is known that high purity alkyl glyoxylate can be isolated from a mixture containing alkyl glyoxylate, water, and alcohol in combined form by distillation from P.sub.2 O.sub.5 (W. Oroshnik and P.E. Spoerri, J. Amer. Chem. Soc. 1941, 63,3338). Product losses are high and a highly corrosive waste stream is formed which poses difficult disposal problems. In addition, the reaction with P.sub.2 O.sub.5 is extremely exothermic. Consequently, this technique is suitable only for small-scale operations.
U.S. Pat. No. 4,356,071 discusses treating anhydrous trioxane or formaldehyde with microwave radiation prior to polymerization to form thermally stable polymers. This process is not directed toward product purification.
The above-described methods of product purification are uneconomical, due to the high cost of multiple distillation stages, high product losses and, in the case of treatment with P.sub.2 O.sub.5, the additional problem of disposal of corrosive waste. It would be desirable to have an economical method of product purification which decomposes the hemiacetal byproducts to increase product yield. It is also desirable to have a more efficient method for producing high purity alkyl glyoxylate with a low water content Such an economical process would involve the removal of water and alcohols and thus prevent the formation of hemiacetals and hydrates.